Self Destruction
This is a major overriding theme of the film. Each character in their own way self-destructs. Kane commits suicide to escape the humanoid, Ventress is ready to lose her life as she knows she is already dying, Anya destructs into paranoia believing that everyone is out to get her, Josie destructs by no longer caring what happens to her, and Lena fights to keep herself alive. Thus, the entity dying from the grenade that was attempting to mimic Lena represents her defeating that part of herself that desires to self-destruct.
Mutation
The Shimmer is an anomaly that is expanding rapidly and set to take over the earth. It is how the creatures and plants become different or mutate that is most fascinating in this film. The reason is that the mutation seems to relate to the state of the individual or the animal, plant, et al. Thus, when something mutates it becomes more of what is dominant within it.
Salvation
Although opposite of the title, salvation is a major theme of the film. We see it as the sole objective for Lean: to save her husband, and by attempting to do so she is set in course to save the entire planet from the mutation that is "The Shimmer". It is because she believes her husband can be save that she is able to overcome so many obstacles in her path.
